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Monitor, display and log up to six sensors and display up to 10 readings! Pt.1: By MAURO GRASSI Multi-Purpose Car Scrolling Display This project started out as a digital dashboard display but has grown and can be used in any measurement or data logging application where you have 9-12V DC available. It can monitor up to six signals and display up to 10 computed values in a scrolling or static readout on a 7 x 15 dot matrix LED display. S O WHAT’S A SCROLLING DISPLAY? You really need a short video to show what this project does. The readout continually “scrolls” from left to right, displaying one, two and up to 10 computed values from up to six different signals. Each value is pre28  Silicon Chip ceded by its description, such as battery voltage, temperature, duty cycle and so on. If you want to focus on one reading, pressing the sole pushbutton will make the display static. Anyway, let’s just give a sample of what this project can do: • Measure Engine Temperature – have a relay switch on above a preset temp­ erature. • Measure Fuel Injector Duty – have a relay switch if the duty cycle is too high or too low. • Measure Engine RPM – have a relay siliconchip.com.au BATTERY SENSE Pin 1 of CON1 7 x 15 DOT MATRIX DISPLAY LDR SENSE Pin 13 of CON6 VIA CON6 SIX ADC INPUTS FOUR ANALOG/ RESISTANCE INPUTS CON3 TWO FREQUENCY/ DUTY CYCLE INPUTS CON2 S1 Pin 7 of CON6 (PUSHBUTTON) PIC 18F4550 MICRO CONTROLLER TWO CCP INPUTS USB PORT TWO RELAYS/ BUZZERS CON4 TWO DIGITAL OUTPUTS Fig.1: block diagram of the Car Scrolling Display. A PIC 18F4550 microcontroller is at the heart of the project. It processes various inputs, drives the dot matrix display, manages the USB connection and drives the two outputs. switch on at a preset RPM (perhaps to indicate when to change gear). • Measure Throttle Position and Delta Throttle Position – if the accelerator pedal is pressed too abruptly, a relay can be made to switch on this condition. • Measure Speed – have a relay switch if the speed is too high or too low. • Measure Fuel Tank Level as a percentage of full tank – have a relay switch on or off if the level is too high or too low. • Measure Battery Voltage – have a relay switch on if the voltage is too high or too low. • Measure Air/Fuel Ratio – have a relay switch on if the mixture is too rich or too lean. • Measure Cabin Temperature – switch on a fan via a relay if it is too high. • Measure almost any signal coming from the ECU. So pick any six of the above possibilities and that is what this project could do in your car. But that is just for applications involving cars. In reality, this project can be used anywhere where a DC supply from 9-12V is available or you have a computer with a USB port. It accepts voltage, resistance, frequency or duty cycle inputs and has two digital outputs for switching on limit conditions. We will siliconchip.com.au bet that you can think up lots more potential applications. The project itself uses two PC boards stacked with red Perspex on top. The top (display) board has a group of three 7 x 5 dot matrix displays, a USB port and a single pushbutton. The main (lower) PC board has the microcontroller and all the supporting circuitry for the connections and the optional output connections to relays or buzzers. To build and set it up, you will need a laptop or desktop computer with a spare USB port. You will use Windows-based software (downloadable from www.siliconchip.com.au) to set the measurement functions, calibrate the sensors and do data logging. The LED display can be dimmed (either automatically by sensing the ambient light level or manually) and you can select the scrolling speed of the display, as well as the names of the measurements and their units. In static mode, the LED readout can display up to four digits. It can also be turned off using the front panel pushbutton. The two output channels can drive external 12V relays directly and can be programmed to respond to maximum and minimum settings for any of the measured variables. Alternatively, the outputs could drive buzzers to give an audible indication that signals have exceeded their programmed limits. You can choose different sounding buzzers to indicate maximum or minimum conditions, when using two different buzzers. Or you can use only one buzzer and the maximum and minimum limits are indicated by different sequences of beeps. When you only need a visible indication of a limit condition, there are visible cues (a flashing display for a minimum condition and an inverted display for a maximum condition) on the LED display when in static mode. So there are many uses for this display and it’s really up to you as to how you set it up. User operation User operation of the Car Scrolling Display has been kept deliberately simple. There is just one pushbutton on the front panel (S1), a momentary SPST switch. The firmware recognises a short press and a long press. A short press is anything less than about a second, while a long press is anything more than that. There are three display modes. You switch to the next display mode by holding S1 pressed for more than a second, ie, by making a long press. The first is the Scrolling Mode where only the selected reading is continuously displayed as a scrolling string. In this mode, pressing S1 for less than a second (ie, a short press) will take you to the next reading, and that will then scroll continuously. After you have scrolled to the last December 2008  29 The unit is built on two PC boards – a main board and a display board. These are stacked together, along with a red Perspex panel for the dot matrix displays (assembly details next month). Note that the boards shown are prototypes and the final versions are slightly different. reading, making a short press will turn the display off. The sequence can then be repeated. The second display mode is the Static Mode. In this mode, the selected reading is displayed without scrolling. You can make a short press to go to the next reading. Again, making a short press after the last reading turns the display off. The sequence then repeats again. The third and last display mode is the All Scrolling Mode. In this mode, all readings are displayed as a scrolling string. The string then repeats continuously. Pressing S1 while in this mode takes you to the first display mode again and the whole sequence repeats from there. In both scrolling modes, the name of the variable, the value and the unit are displayed as a scrolling string. In Static Mode, up to four digits are displayed at once. In Static Mode, a maximum condition is indicated by the display flashing every second or so between normal and reverse modes, ie, all the normally lit dots become unlit, and vice versa – Fig.5. This is a very dramatic mode to indicate a problem condition. A minimum condition, on the other hand, 30  Silicon Chip is indicated by a flashing reading. As indicated, these visual cues are only available in Static Mode. Note that the Battery Voltage is always displayed first. For each of the displayed variables, you select the variable number and the value index to display. You also set the order in which they are displayed. Remember that you can change all settings and perform the required calibration using a laptop and a USB cable. Electrical signals in cars To get a good understanding of the signals used in cars, you will need to refer to the SILICON CHIP publication “Performance Electronics for Cars”. This has a range of useful electronic projects for cars and also explains how to intercept the signals from your car’s ECU. All modern cars have an ECU (Electronic Control Unit) that manages the ignition timing and fuel injection. Almost all electrical sensors in your car produce a voltage or vary their DC resistance, depending on the quantity being measured, or produce a digital signal (varying the frequency or duty cycle) to indicate the reading. Different sensors have different voltage ranges. For example, a narrowband air/fuel sensor may have an output in the 0-1V range, whereas a tachometer sensor output may be a square wave at 5V with the frequency of the signal proportional to the engine’s RPM. By contrast, a fuel injector signal is digital (12V amplitude), with the positive period (ie, the time the signal is at a high level) normally proportional to the time the injectors are firing. Alternatively, it may be inverted, with the negative period indicating the firing of the injectors. Since all calibration is done in software, either negative or positive duty cycles can be monitored. This project will accept all of these types of signals and with software calibration via the USB port, it is easy to adapt to a wide range of different sensors. How it works The block diagram of Fig.1 shows the main features of the circuit. As you can see, a microcontroller is the heart of the project and it drives the dot matrix displays, manages the USB connection and drives the two outputs. siliconchip.com.au Fig.3 shows the circuit of the main board while Fig.4 shows the circuitry of the display board. In Fig.3, IC1 is the PIC18F4550 microcontroller and there are four multi-way terminal blocks. CON1 (4-way) provides the connections to the battery or DC supply. The 12V input from the car’s battery is passed through a 10Ω 1W resistor and a reverse polarity protection diode (D1). The 10Ω resistor will normally drop around 2V since the circuit typically draws around 200mA, depending on the display brightness and the number of lit pixels. A 16V zener diode (ZD1) clamps the input voltage in case of transients. This is necessary to protect both the input supply bypass capacitor (470μF, 25V) and the 3-terminal low-dropout regulator REG1 (a LM2940-5). The entire circuit runs from the +5V rail output by REG1. This supply rail is bypassed by a 47μF 16V capacitor and the 100nF monolithic capacitors near the microcontroller and the other logic ICs. CON2 (4-way) accepts the two identical frequency/duty cycle inputs. Considering pin 2 of CON2, for example, the signal is applied to the base of NPN BC337 transistor Q19 through a 33kΩ resistor. The 10kΩ resistor to ground sets the switching threshold to around +2.6V. That is, the transistor switches on when the signal input is above +2.6V and switches off for voltages below that. Diode D5 clips any negative voltage excursions of the signal to the base of the transistor to around -0.6V. The collector output of the transistor is pulled up by a 10kΩ resistor and is fed to the CCP1 (Capture/Compare) input (pin 17) of IC1 via a low-pass filter composed of a 1kΩ resistor and a 10nF capacitor. This low-pass filter removes potentially noisy signal transitions. The frequency and duty cycle of the input signal is measured by capturing the value of an internal timer run from the microcontroller’s system clock (12MHz). It counts how many system clock ticks occur when the signal is low and when the signal is high. TYPICAL DISPLAY READOUTS siliconchip.com.au The counter is 24 bits wide. For example, when applying a 40% duty cycle rectangular wave at 100Hz, we will obtain the following counter values: CHigh = 48,000 and CLow = 72,000 In other words, the internal timer running from 12MHz counts up to 48,000 in the time that the signal is high and up to 72,000 in the time the signal is low. From these two values, the firmware calculates the frequency and duty cycle as follows: Freq = 12,000,000/(CHigh+CLow); and Positive Duty Cycle = 100CHigh/(CHigh+CLow) Voltage/resistance inputs The four voltage/resistance inputs are connected to the 6-way connector CON3. Each analog input passes through a voltage divider consisting of 22kΩ and 10kΩ resistors and bypassed by a 100nF capacitor. Each resulting voltage is then digitised by the microcontroller using the onboard ADC (analog-to-digital converter) which has 10 bits of resolution and whose full range is from 0-5V. The division factor from the 22kΩ and 10kΩ resistors is 3.2 which means that the analog inputs have a full range of 0-16V, suitable for most applications in a car or any vehicle with a 12V battery. Any voltages above 16V will not be correctly read (ie, readings will plateau), because the input protection diodes on the ADC inputs of IC1 will begin to conduct. The high series input impedance will ensure that the input Fig.2: these diagrams illustrate some of the readouts that can be scrolled across the three 7 x 5 dot matrix displays. The battery and ambient light functions are built in, while all other functions are set-up by the user via a PC program. December 2008  31 +5V 100nF 100nF 11 32 Vdd Vdd AN5 10k 1k CON2 FQ1 FQ2 GND 33k 2 C B K 3 10k 1 E D5 Q19 BC337 17 47 µF 16V 1k CCP1 RE2 RE1 100nF 8 10k 10 9 10nF D7 MCLR/Vpp K 1 A A +5V PGD/RB7 PGC/RB6 10k 1k 33k B K 10k C E D6 Q18 BC337 16 IC1 PIC18F4550 10nF RA4 RB5 RB4 RB3 RB2 RB1 CON3 AN3 AN2 AN1 AN0 GND 1 2 RB0 4 x 22k 5 3 4 4 3 5 2 6 RD7 AN3 RD6 AN2 RD5 AN1 RD4 AN0 RC7 RC6 10k 100nF 10k 100nF 10k 100nF 10k 100nF RD2 RD0 D+ D– RC0 13 X1 20MHz 22pF RD1 OSC1 AN4 RD3 14 22pF OSC2 VUSB Vss 12 SC 2008 CAR SCROLLING DISPLAY 39 CCP2 A +5V 40 Vss 31 6 38 37 36 35 34 33 30 29 28 27 26 25 21 19 24 23 15 20 7 22 18 1 µF 16V 1 µF 16V 10k MAIN BOARD Fig.3: the main board circuitry. PIC microcontroller IC1 accepts the various analog and frequency input signals, processes these signals and then drives the separate display board via connector CON6. itself is not damaged. The downside of having a large dividing factor of 3.2 (16V = 5V x 3.2) is that you lose resolution in the ADC conversion. Since the ADC is 10 bits 32  Silicon Chip or 1024 levels, we obtain a value of 16V/1024 or about 16mV sensitivity. While this is plenty for most applications, you can increase the sensitivity of the input if you know in advance that your sensor has a nominal output much lower than 16V. This involves changing the 22kΩ resistor on the corresponding analog input. The following equation is used siliconchip.com.au D1 REG1 LM2940-5 OUT K IN GND A 10 Ω 1W +12V 100nF A 56k ZD1 16V 1W 2 1 (BATTERY SENSE) CON4 A 1k C B 2 A Vpp GND 6 1k C B AUX 4 PGD 5 PGC RLY1 1 D3 Vdd 3 RLY2 3 2 K K (ICSP) 1 4 D2 Q16 BC337 E +5V 4 3 K 470 µF 25V CON1 E Q17 BC337 220 µF 50V TO CON7 ON DISPLAY BOARD CON5 CON6 +5V 6 24 23 1 2 3 4 5 7 22 8 9 10 11 12 14 21 26 20 16 25 27 13 17 K D4 A 1.5k A 18 19 GND K D1-D3: 1N4004 BC337 A B E A to get an approximate value for the resistor: R = 2000V - 10,000 where V is the maximum voltage range required (>5V) and R will be the new siliconchip.com.au GND K ZD1 C LM2940-5 IN K GND Oxygen sensor loading Although the ADC inputs of IC1 have a high input impedance, the load on the analog inputs will be the sum of the 22kΩ (or your replaced value) resistor and the 10kΩ resistor, ie, 32kΩ (or 10,000 + R). While this loading is high enough to result in very small current draw from most sensors in your car, you should be aware that typical narrowband oxygen sensors do not tolerate more than about 10μA current load. Since the ECU will have its own current load, we should aim to draw no more than about 1μA extra from such a sensor. This means that if you wish to connect an oxygen sensor to this project, you should omit or remove the 10kΩ resistor to ground on the corresponding analog input. The result will be that the loading will then be the series impedance of the 22kΩ resistor and the high input impedance of IC1’s ADC input. The resulting extra current should be less than 1μA since the ADC inputs have a typical leakage current of just 500nA. Note that there will also be negligible transient loading due the 100nF capacitor. Additional input channels 15 D4-D6: 1N5819 D7: 1N4148 sensitivity will be about 6mV and the resistor value will be 2kΩ. Since all calibration is done in soft­ ware, you only need to replace the 22kΩ resistor corresponding to your analog channel to improve the accuracy for that channel. The software does not need to be changed, as the values will be correct for your new divider when you perform the next calibration. OUT resistor value (ie, to replace the existing 22kΩ resistor). The resulting sensitivity will be approximately the value of V in mV (millivolts); eg, if V = 6, then the There are two additional analog channels used. One is used to measure the battery voltage at pin 1 of CON1. It has its own 56kΩ and 10kΩ voltage divider and 100nF bypass capacitor. The other analog channel is used to monitor a voltage divider on the display board consisting of a light dependent resistor (LDR1) and an 82kΩ resistor. The analog signal is at pin 13 of CON6 and is used to measure the ambient light level, to vary the brightness of the LED display. CON4 is used to connect the relays and/or buzzers used for the limit conditions. Each digital output from the microcontroller is applied to the base of an December 2008  33 CON7 3.3 +5V Q1 6 470F 16V B 15 x 680 1 Q2 E Q7 E B C +5V E B C Q8 Q15 E B C B C E C 2 3 4 21 5 22 23 9 24 10 25 26 8 27 C1-C15 1 17 16 20 19 12 1k 12 11 13 Sin 16 1 16 Vdd Q0 Q1 Q2 Q3 MR 14 CK C15 9 100nF 10 14 C7 +5V G 11 SER C8 4 C2 C1 USB TYPE B SOCKET 1 2 3 3 IC2 Q4 4 74HC595 5 4 Q5 6 Q6 7 Q7 9 So LCK SRCK OE 15 2 15 12 LED ARRAY 3 B +5V E C Q1 – Q15: BC327 11 6 Vss 10 7 LDR 13 LED ARRAY 2 13 5 8 LED ARRAY 1 14  LDR1 S1 IC3: ULN2003 8 S1 7 82k 1k GND 15,18 SC 2008 CAR SCROLLING DISPLAY DISPLAY BOARD Fig.4: the Display Board circuit. It uses a 74HC595 shift register (IC2) to drive the rows of the three dot-matrix LED arrays via a ULN2003 Darlington array (IC3). Transistors Q1-Q15 switch the display columns. NPN BC337 transistor (Q16 or Q17) via a 1kΩ resistor. Each transistor is configured as a switch, to drive the coil of the relay or a 12V buzzer. Diodes D2 & D3 clip any back-EMF spikes generated when the relays switch off, while the 220μF 50V capacitor is used for bypassing. The microcontroller (IC1) runs from a 20MHz crystal and the two 22pF ceramic capacitors provide the correct loading. The 1kΩ resistor from the 5V 34  Silicon Chip rail is used to pull up the MCLR-bar input (pin 1) of the microcontroller (this is the active low reset input). The microcontroller is reset by internal POR (power on reset) circuitry. CON5 is optional unless you fancy doing your own programming using the PicKit2 programmer from Microchip. We used this during development of this project. You will not normally need to use this connector. There is one further sub-circuit on the main board, consisting of a Schottky diode D4 and two resistors (10kΩ and 1.5kΩ). Pin 17 of the 27-pin connector CON7 is the VUSB rail (ie, positive power from the USB port on the display board). This will be around +5V when a USB cable is connected and 0V otherwise. This input passes through the voltage divider consisting of 1.5kΩ and 10kΩ resistors. The division factor is thus 1.15 meaning that pin 22 of IC1 siliconchip.com.au will be at around 4.3V when a USB cable is connected and at 0V otherwise. This pin is configured as a digital input (bit 3 of PORT D) which allows the firmware to detect when a USB cable is connected or disconnected. Schottky diode D4 allows the circuit to be powered directly from the USB port and connects directly to the +5V rail. In the worst case, the VUSB line will be at +4.75V (5V ±5% is what the USB standard specifies) and so the +5V rail can be as low as +4.5V when powered directly from the USB port. D4 also protects against reverse polarity and prevents current flow into the USB port when the circuit is powered from a 12V battery or power supply. Because the +5V rail can be substantially lower than +5V when powered from the USB port, you MUST perform any calibration with the full 12V input from the car battery. The actual voltage of the +5V rail will affect the ADC readings from the analog channels because it is the positive reference for the ADC conversion. This will be explained in the calibration instructions, next month. Display circuit Microcontroller IC1 controls the display via 27-pin connector CON6, which plugs into CON7 on the display board – see Fig.4. Fifteen of these lines control BC327 PNP transistors to drive the columns of the LED display. The display board consists of three dot matrix LED modules, a 74HC595 shift register (IC2) and a ULN2003 Darlington driver (IC3). The display is multiplexed, meaning that only one column is lit at any one time. The brightness of the display is varied by changing the duty cycle of the column driving signals. The display refresh frequency is around 150Hz. IC2 is an 8-bit shift register and the seven least significant bits (Q0-Q6) are used to drive the seven rows of the display. The microcontroller uses three lines – SER (data input), G-bar (output enable) and CK (clock) – to load each row value into IC2. The G-bar (enable) line forces all outputs of the shift register to go tri-state. This effectively blanks the display. This is done by the microcontroller when the display is being refreshed or when the shift register is being loaded. The time that the display is disabled is so short it is impercepsiliconchip.com.au Main Features & Specifications • • • • • • • • • • • • • • • • • • • • • Can be powered from 9-12V DC or from a USB port (5V). Two Frequency/Duty Cycle Inputs with frequency up to 10kHz. Positive Duty Cycle Range: 0-100%. Four Voltage/Resistance Inputs Plus Battery Voltage (the latter has its own channel). Voltage Range: 0-16V (greater or smaller ranges possible by changing one resistor). Sensitivity with 16V scale: approx. 16mV. Best Sensitivity: approx. 5mV (requires changing one resistor and recalibrating using the supplied PC software). Two output channels to drive external relays or buzzers. Up to 10 displayed variables. Averaging or direct acquisition mode for each variable. Screen dimming on ambient light with adjustable sensitivity and selectable minimum brightness. 7 x 15 dot matrix LED display (scrolling or static display). Static display of up to 4 digits (floating point) Selectable scrolling speed. On screen limit warnings for each variable in the static display mode. Software calibration using polynomial interpolation. Persistent settings stored in non-volatile memory. Easily load and store previous settings to file on your computer. Easily load and store different calibration point files on your computer. All settings changeable using the USB port and PC host program. Data logging via the USB port; selectable variable update frequency from 0.1-8Hz; can collect 1000s of samples to a PC’s hard drive. tible. The SER (data) line feeds the data into the shift register and is also controlled by a simple digital output of the microcontroller. The seven bits from the shift register are used as inputs to the ULN2003 Darlington array (IC3). The ULN2003 can sink up to 500mA in total between its seven outputs. Note that there are no current limiting resistors to the displays. Instead, we rely on the beta limiting of the transistors via the 680Ω base drive resistors. We found that even smallvalue limiting resistors markedly decreased the perceived brightness of the LED display. However, we have included a 3.3Ω current-limiting resistor on the supply rail to the entire display board. Because the display can draw substantial currents (up to around 300mA peak), thereby affecting the +5V rail used for the positive reference to the ADC system, the firmware also turns off the display when digitising the analog Fig.5: in-range measurements appear as shown at left, while out-of-range measurements alternate between normal and reversed mode (top right) when above maximum or flash on and off when below minimum. inputs. This happens too quickly to be perceptible. An additional digital input on IC1 is used for pushbutton switch S1. It will be high when S1 is pressed and low otherwise. The signal is fed via CON6 at pin 7 and the switch is de-bounced by the software. The USB type B socket is on the display board and the four connections are fed to the main board via CON6. That completes the circuit description. Next month we give the full constructional details and set-up proSC cedure, as well as the parts list. December 2008  35